1. Field of the Invention
The present invention generally relates to a radiation inspection apparatus that uses an ionizing radiation such as an X-ray, a beta radiation, and a gamma radiation. More specifically, the present invention relates to a radiation inspection apparatus including a reference detecting element, which corrects an intensity distribution and an intensity variation of an ionizing radiation, which has been radiated from a radiation source, based on an intensity of the ionizing radiation detected by the reference detecting element.
Priority is claimed on Japanese Patent Application No. 2009-167875, filed Jul. 16, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, there has been performed a radiation inspection, which detects an X-ray or a soft X-ray, for example, in various fields of research or inspection of foods, industrials, medical techniques, securities, and so on. In the fields of inspection, it is determined whether or not there is a certain object or a defect in an inspection target by watching a gray image that is relatively sharp. In a specific field of industrials, medical techniques, and so on, the determination may be performed based on the gray image, which gray is relatively light, by a doctor or a laboratory technician. The doctor or the laboratory technician watches the gray image using a display monitor, and performs the determination comprehensively and selectively, considering various elements such as an uneven brightness of the display monitor, an uneven brightness of an image pick-up system, and a trend or a visible characteristic that is peculiar to the inspection target.
On the other hand, if the determination is performed by an auto inspection, not by a human's visual inspection, then various corrections are necessary such as a correction of the uneven brightness of the image pick-up system, a correction of a detection sensitivity, a correction of the trend that is peculiar to the inspection target, an emphasis of a gray value of the gray image, and an image processing that effectively extracts a characteristic of parts of the inspection target.
In the descriptions below, an ionizing radiation will hereinafter be referred to as an X-ray for an example. In the inspection using the X-ray, the gray image is acquired by setting the inspection target such as a product and a body between an X-ray source and a detecting apparatus and by detecting the intensity of the X-ray based on a transmissivity of the X-ray, in general. Therefore, the stability of the X-ray source is very important. If the intensity of the X-ray source is changed or losing stability, then the instability of the intensity of the X-ray source influences a detected image or the accuracy of the detection directly.
To make the X-ray source drive in stable, a feedback control, a temperature control, and a monitoring of an X-ray value are performed. Depression of the X-ray value of the X-ray that the X-ray source irradiates cannot be neglected, for an X-ray tube used in the X-ray source is short-lived. For example, Japanese Unexamined Patent Application, First Publications, No. 2001-198119 discloses a method of monitoring the X-ray value and feed back the monitoring result to a measurement system.
FIG. 6A is a view illustrating a configuration structure of a reference detecting element in an X-ray inspection apparatus disclosed in Japanese Unexamined Patent Application, First Publications, No. 2001-198119. The X-ray inspection apparatus includes an X-ray source 3, a rotary part 2, an X-ray detector 11, and a reference detecting element 12. The rotary part 2 includes an opening 2h where an inspection target 9 is set. The reference detecting element 12 detects the intensity of the X-ray irradiated from the X-ray source 3 so as to correct the intensity variation of the X-ray irradiated from the X-ray source 3. The reference detecting element 12 is disposed on a line segment L that connects a focus of the X-ray source 3 and the outer periphery of the opening 2h of the rotary part 2 or outside of the line segment L, and is disposed on an extension circular arc of an alignment circular arc of the detecting element in the X-ray detector 11.
The length of the X-ray detector 11 is adjusted so that both ends of the X-ray detector 11 are disposed on the line segment L that connects the focus of the X-ray source 3 and the outer periphery of the opening 2h of the rotary part 2 or outside of the line segment L. The reference detecting element 12 is disposed on the both ends of the X-ray detector 11.
The X-ray is irradiated from the X-ray source 3 and detected by the X-ray detector 11 and the reference detecting element 12. The X-ray that enters the reference detecting element 12 passes through the outside of the opening 2h. The inspection target 9 is not disposed on a path of the X-ray from the focus of the X-ray source 3 to the reference detecting element 12. Therefore, the inspection target 9 does not interfere with the detection by the reference detecting element 12. The distance between the focus of the X-ray source 3 and the reference detecting element 12 is equal to the distance between the focus of the X-ray source 3 and the X-ray detector 11. Therefore, the quality of the X-ray that enters the reference detecting element 12 is equal to the quality of the X-ray that enters the X-ray detector 11.
FIG. 6B is a block diagram illustrating a configuration structure of the reference detecting element in the X-ray inspection apparatus disclosed in Japanese Unexamined Patent Application, First Publications, No. H11-128217. The X-ray inspection apparatus includes an X-ray tube 1, an X-ray detection unit 20, a calculation unit 23, a memory 24, a display unit 25, and an X-ray control unit 26. The inspection target 9 is disposed between the X-ray tube 1 and the X-ray detection unit 20. The X-ray is irradiated from the X-ray tube 1 to the inspection target 9. The X-ray tube 1 and the X-ray detection unit 20 are integrated to be rotated, so that X-ray scanning data of the inspection target 9 can be acquired in the direction of 360 degrees or 180 degrees.
The X-ray detection unit 20 includes a collimator 21 and an X-ray detector 22. The collimator 21 collimates an incident X-ray. The X-ray detector 22 detects the X-ray. The collimator 21 and the X-ray detector 22 are in a shape of circular arc having its center at the X-ray tube 1. The X-ray detector 22 includes channels of X-ray detecting elements which are disposed in a shape of circular arc having its center at the X-ray tube 1. The number of the channels of the X-ray detecting elements is from 500 to 1000. The X-ray detecting element includes a scintillator element, which converts the X-ray to a light, and a photodiode that detects the light, which has been converted by the scintillator element, to output an electrical signal. The scintillator element and the photodiode are not illustrated in the figures. The X-ray detector 22 includes imaging X-ray detecting channels 22a, reference channels 22b, and monitoring X-ray detecting channels 22c. The reference channels 22b and the monitoring X-ray detecting channels 22c are disposed at both ends of the X-ray detector 22.
The reference channel 22b is for measuring the intensity of the X-ray from the X-ray tube 1 that has not transmitted through the inspection target 9. The monitoring X-ray detecting channel 22c is for detecting the movement of the focus of the X-ray tube 1.
The calculation unit 23 calculates data for generating X-ray image based on a detection value by the X-ray detection unit 20. The calculation unit 23 outputs a command signal to each unit of the X-ray inspection apparatus including the X-ray control unit 26. The display unit 25 displays the X-ray image such as a cross-sectional image.
In the above examples, the X-ray inspection apparatus is supposed to be the X-ray CT apparatus. Therefore, the distances between the X-ray source and the detecting unit are the same. FIG. 7 is a perspective view illustrating the configuration structure of the X-ray inspection apparatus when an inspection target 32 is sheet-shaped and is measured by a measurement X-ray line sensor 31 disposed in planate. In this case, the inspection target 32 is a paper or a sheet that is produced by a line production for an industrial use. The width of the inspection target 32 is a few meters, for example. The X-ray is irradiated from an X-ray source 30. The X-ray is transmitted through the inspection target 32 and detected by the measurement X-ray line sensor 31 disposed in planate.
In the industrial use, the inspection target 32 is transmitted through a previous process, an inspection process, and a post process by the line production in planate. Therefore, it is difficult to make only the inspection apparatus arranged in the shape of circular arc.
If the planate line sensor is used, then the distance between the X-ray source and the inspection target at a center portion are different from the distance between the X-ray source and the inspection target at a peripheral portion. Similarly, the distance between the detecting unit and the inspection target at a center portion are different from the distance between the detecting unit and the inspection target at a peripheral portion. The X-ray, especially the soft X-ray, is influenced by air absorption, temperature, humidity and atmosphere. The X-ray near the reference unit, which is distant from the detecting unit, is inclined to be different from the X-ray near the detecting unit. The incident angles of the detecting unit may be variable.
The X-ray is radically diffused from the X-ray source. The reference detecting unit, which is disposed near the peripheral portion, is away from the measurement detecting unit, which is disposed near the center portion. Therefore, it is not suitable to correct the X-ray value of the measurement detecting unit based on the X-ray value of the reference detecting unit. The correction is performed by estimating the change of the X-ray value of the measurement detecting unit from the change of the X-ray value of the reference detecting units, which are disposed at both ends of the measurement detecting unit, under the assumption that the distribution of the X-ray from the X-ray source does not change.